1. Field of the Invention
The present invention relates to a radiation generating apparatus having a radiation tube in an envelope filled with an insulating liquid and to a radiation imaging apparatus using the radiation generating apparatus.
2. Description of the Related Art
A radiation generating apparatus in which an electron source and a target are placed in a radiation tube and an electron emitted from the electron source is irradiated to the target, thereby generating a radiation has been known.
In order to generate a radiation suitable for radiation imaging, it is necessary that a high voltage of 40 to 150 kV is applied between the electron source and the target and an electron flux is accelerated to high energy and irradiated to the target. Thus, a high potential difference of tens of kV or more is caused between the electron source and the target and between the radiation tube and the envelope enclosing the radiation tube. Therefore, in order to stably generate the radiation for a long time, it is required that the radiation generating apparatus has withstanding voltage performance (voltage proof performance) at such a high voltage.
The Official Gazette of Japanese Patent Application Laid-Open No. S61-066399 discloses a rotary anode X-ray tube apparatus in which a cooling insulating oil is filled between a rotary anode X-ray tube and an inner wall of an envelope, thereby assuring the voltage proof performance. By allowing the cooling insulating oil to smoothly flow between the rotary anode X-ray tube and the envelope, a sludge which is deposited onto the surface of the rotary anode X-ray tube is prevented, thereby reducing a discharge between the rotary anode X-ray tube and the envelope.
However, according to the related art, there is a case where a discharge occurs between the rotary anode X-ray tube and the envelope through an outflow inlet for allowing the cooling insulating oil to flow and an X-ray emitting port of the rotary anode X-ray tube. There is also such a problem that if the X-ray tube is damaged by the discharge, the X-ray cannot be stably generated for a long time.
As a measure for solving such a problem, a method whereby a layer of the cooling insulating oil between the rotary anode X-ray tube and the inner wall of the envelope is sufficiently thickened is considered. However, the withstanding voltage performance of the insulating liquid such as a cooling insulating oil or the like is more influenced by an electrode shape, an electrode surface smoothness, a temperature, impurities, convection, or the like as compared with that of another insulating member. Therefore, a thickness of the layer of the cooling insulating oil between the rotary anode X-ray tube whose temperature becomes a high temperature of 200° C. or higher during the driving and the inner wall of the envelope has to be set to a thickness enough to avoid the discharge. Thus, the envelope increases in size and a size and a weight of the whole X-ray generating apparatus increase. When the cooling insulating oil layer is thickened, an attenuation amount of the X-ray at the time when the X-ray passes through the cooling insulating oil layer increases. A higher voltage, a larger current, and the driving for a longer time are necessary in order to compensate such an attenuation amount, so that it causes an increase in electric power consumption.
The foregoing problem is not limited to the reflection type radiation generating apparatus but there is also a similar problem in a transmitting type radiation generating apparatus. Therefore, in both of the reflection type and the transmitting type, it is required that a distance between the radiation tube and the envelope is shortened as much as possible to thereby miniaturize the apparatus, such a withstanding voltage as to make it difficult to cause a discharge between the radiation tube and the envelope is assured, and the attenuation amount of the radiation is also decreased.
It is, therefore, an object of the invention to provide a radiation generating apparatus having a construction in which a radiation tube is provided in an envelope filed with an insulating liquid, wherein a miniaturization of the apparatus, an improvement of a withstanding voltage between the envelope and the radiation tube, and a decrease in attenuation amount of the radiation are realized and to provide a radiation imaging apparatus using the radiation generating apparatus.